The present invention relates to water filtration systems and apparatus, particularly those for removing iron and manganese from underground water.
While there are a variety of techniques and processes for iron and manganese removal from water sources, most removal processes can be generalized into three major categories: oxidation followed by solids/liquid separation; ion exchange; and coordination (sequestration and/or stabilization). See for example, Sung, W. and Forbes, E. J., Some Considerations On Iron Removal, Journal of Environmental Engineering, Volume 110, No. 6, December, 1984. Sung and Forbes note the most common control technique may be chlorination followed by filtration, which falls under the first category. They note, however, that recent advances link the formation of high levels of trihalomethanes with prechlorination, which may necessitate a modification of this approach, favoring aeration or oxidation.
Prior oxidation processes for iron removal from aqueous systems include oxidizing ferrous (Fe.sup.2+) iron to ferric (Fe.sup.3+) iron (i.e., changing a water-soluble Fe.sup.2+ into water-insoluble Fe.sup.3+), and then removing Fe.sup.3+ in a large filtration pond, where the insoluble iron settles. In these traditional processes, raw water is typically aerated (or an oxidizing agent added) before entering a large oxidation pond wherein a slow oxidation reaction (spontaneous oxidation) occurs. However, the oxidation rate is rather slow for such an iron removal process, since the procedure is largely a physical settling of the insoluble iron.
Attempts have been made to improve on this process. One apparatus includes adding air to water containing ferrous iron, organic matter, and H.sub.2 S. See Japanese patent document 51-102343. The water-air mixture passes through a filtrating layer such as limestone, carbon, plus iron oxyhydroxide (FeOOH) as an optional catalyst. In this system, the incoming raw water is saturated with oxygen, the filtrating layer always operating under aerofillic conditions. French patent 77 35994 (Foessel) discloses a gravity filtration system which eliminates iron, manganese, and calcium from water. A first filter bed includes sand of various particle sizes which removes most of the iron and some of the manganese, as well as organic compounds. A porous element contactor exists between a sand filter and a carbon filter. Manganese remaining after passing through the sand filter deposits at the outlet of the porous element contactor. Water free of iron, manganese, and calcium is obtained after the water passes through the carbon filter. Air is injected counter-concurrently into the first sand filter. In this system, solids which form on the top surface of the sand filter, which may include iron oxyhydroxides, are evacuated through a conduit, to keep the sand filter from clogging.
Attempts have been made to explain the wide variation in reported rate constant for oxygenation kinetics of Fe.sup.2+. Sung and Morgan, Kinetics And Products of Ferrous Iron Oxygenation In Aqueous Systems, American Chemical Society, Volume 14, No. 5, May, 1980, studied the effect of ions, alkalinity, and temperature on oxygenation kinetics, and attempted to identify products of oxygenation. Both homogeneous and heterogeneous oxygenation kinetics were studied. The homogeneous oxidation rate constant, at constant pH and oxygen concentration in the water, was found to drop with increasing ionic strength, and to increase with increasing temperature. Chloride ions and sulfide ions reduced oxygenation rate. The heterogeneous oxidation study concluded that autocatalysis is noticeable only around a pH of 7 or above due apparently to the particulate surfaces forming faster and adsorption of Fe.sup.2+ being more favorable at these pH's. Sung and Forbes, mentioned above, noted that one key to removing iron from water is how to remove FeOOH particles, either by flocculation or filtration. They also note that iron removal by filtration is not well understood, noting that silica has been reported to both promote and retard iron removal, and affects the iron precipitates. Sung, Identity And Character Of Iron Precipitates, Journal Of Environmental Engineering, Volume 109, February, 1983, notes that typical iron hydroxide solids have surface area of about 200 m.sup.2 /gm and that the aging process of these solids decreases the surface area and total active sites for Fe.sup.2+ adsorption by a factor of about 10. Japanese patent publication 58-186493 discloses a combination chlorination/oxidation system, followed by filtration to remove iron from underground water. Granulated oyster shell is soaked in NaClO, then water having oxygen dissolved therein is passed through. The treating agent is said to be regenerated by backwashing. Also of interest is WO89-11454 (Partanen), which discloses a counter-current flow of air and water which helps to remove scale from filter surfaces. A chemical or biologic material functions as a first iron removal stage, followed by a filtration screen.
Many of the above methods and apparatus are expensive to operate and complicated to control. The ability of removing iron and manganese from water depends on several factors, such as pH, alkalinity, sulfate ion concentration, soluble silica, etc. Further, even those processes which apparently use iron oxyhydroxide as catalyst exhibit an aging process in which the catalyst surface decreases in total active sites for iron adsorption over time. Still further, most of the above-mentioned systems require extensive backwashing to remove insoluble material.
It would be advantageous to develop an iron and/or manganese removal system which significantly reduces costs of manufacturing and operation, which provides a filter media which does not substantially age or wear out, which does not require a replacement, which is environmentally clean, and in which backwashing is short (less than about 10 minutes).